In relation to fuel tanks for a motor vehicle, it is a conventional measure for a tank venting device to be connected to activated carbon filter as a fuel vapor filter, thereby to ensure that, in a tank refilling procedure, any gases and vapors which escape from the internal volume of the fuel tank are prevented from reaching the ambient atmosphere without first being cleaned of fuel vapors. It will also be appreciated that gases which are given off due to the effect of heat on the tank or due to sloshing and splashing movements of the fuel in the tank are also discharged by a tank venting system. Usually the activated carbon filter which serves as a fuel vapor filter is regenerated by means of combustion air which is drawn in by the engine of the motor vehicle to which the tank is fitted. An activated carbon filter of that nature is usually only of a limited capacity as, for reasons of cost and available space, it is generally desirable for the activated carbon filter to be kept as small as possible. That is only possible if it is subjected to a minimum effect in terms of hydrocarbon-charged gases acting thereon, in other words, if the volume of gas flowing through the activated carbon filter is kept comparatively small.
It will be appreciated further that the flow of gases through the activated carbon filter is at its greatest in a tank refuelling procedure. In such a situation, the fuel tank is frequently required to receive a volume of fuel of between 30 and 60 liters per minute. A corresponding flow by volume of gas or air has to be discharged at the same time. In order in that situation to minimise the loading on the activated carbon, it is possible to connect a recirculation conduit to the filler pipe of the tank, the recirculation conduit providing for recirculation of the gas going to the activated carbon filter through the filler pipe of the tank in such a refuelling operation.
It will be seen therefore that this operational system provides that ambient air which is entrained in the tank refuelling operation by the jet of fuel as it is in the process of issuing from the refuelling gun, before it can pass into the atmosphere again by way of the activated carbon filter, is branched away from the activated carbon filter by way of the recirculation conduit and is passed back through the filler pipe again. In that way the fuel vapor filter in the form of the activated carbon filter is kept free from a part of the gases which are displaced by the fuel passing into the tank.
The amount of gases which is recirculated in that fashion is subject to limits, more specifically primarily by virtue of the diameter of the recirculation conduit. The recirculation conduit normally opens approximately in the region of the discharge opening of the fuel filler gun, in the filler pipe of the tank. The aim at any event in a tank refuelling procedure is to ensure that hydrocarbon-bearing vapors or gases do not escape to the atmosphere from the tank by way of the filler pipe. For that reason, a design of filler pipe provides that it is elastically deformable at least over portions of its periphery and is so designed that it represents a constriction in the filler pipe when in the non-expanded condition thereof. When the motor vehicle tank is being refuelled, the cross-section of the filler pipe is expanded by the jet of fuel impinging thereon so that the gases in the fuel tank are reliably prevented from escaping through the filler pipe during the refuelling procedure. An arrangement of that kind can be found for example in DE 197 16 812 A1 and is referred to as a liquid seal. As an alternative thereto, sleeve-like extension portions can be provided at the discharge end of the filler pipe, which extend as far as the bottom of the fuel tank and are held in a closed condition by the hydrostatic pressure of the head of fuel in the tank.
If the diameter selected for the recirculation conduit is too large, there is the risk that only a part of the gases issuing from the recirculation conduit in the filler pipe is actually also entrained into the filler pipe whereas the remaining part of the gases passes into the ambient atmosphere by way of the filler pipe. That constitutes a reduction in the effectiveness of the liquid seal.